The answer
The next .then receives that value. For example, Promise.resolve(5).then(n => n + 1).then(n => console.log(n)) logs 6. Each value is passed to the next .then.
Promise Chaining: A Deep Dive in JavaScript
What happens if a .then callback returns a value in JavaScript?
More FAQs in Promise Chaining: A Deep Dive in JavaScript
Each .then returns a new promise. If the callback returns a value, the new promise fulfills with it. If it returns a promise, the new promise waits for it. The next .then receives the result. This lets you sequence async operations flatly.
The next .then waits for the returned promise to settle. The next .then receives the resolved value. This is how you chain async operations that depend on each other.
Return an object or array from each .then: .then(user => fetchPosts(user).then(posts => ({ user, posts }))). Or use async/await with local variables, which is cleaner and avoids this problem.
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